PROJECT SUMMARY/ABSTRACT The balance of the gastrointestinal tract and return to a state of equilibrium after times of stress (infectious, chemical or otherwise) is a critical determinant in regulating many human intestinal disorders including those of autoimmune and infectious origins. Perhaps the best studied genetic intestinal disorder is inflammatory bowel disease (IBD), however, the exact etiology of IBD is not fully understood. In order to better understand intestinal immune homeostasis, we carried out a forward genetic screen to unbiasedly identify these essential genes with non-redundant functions. To date, we have used this method to examine 49,690 mice, which serve as a reservoir for 104,845 mutant alleles in the coding or splicing of 19,368 genes. Susceptibility to experimental colitis results from mutations in genes in a number of broad functional categories, including classical immune genes, growth factors, and extracellular matrix proteins as well as from mutations in genes with less expected functions such as vesicular trafficking and metabolism. We have chosen to examine a subset of genes (Smcr8 and Wdr41) that cause a striking phenotype and exist along with C9orf72 in the same macromolecular complex linked to vesicular transport. The Smcr8-Wdr41-C9orf72 (SWC) complex is a tripartite guanine nucleotide exchange factor that stands at the intersection of multiple human disease pathways, inflammatory signaling, autophagy, and lysosomal transport. In this proposal, we will examine the components that mediate the cellular and molecular dysfunction in SWC complex deficiency and the resulting pathology in three Specific Aims: (1) To examine cellular components requiring the SWC complex for regulating intestinal and peripheral inflammation. (2) To understand the molecular mechanism, signaling pathways and in vivo consequences of endosomal TLR dysregulation in SWC deficiency. (3) To examine the mechanism by which the SWC complex regulates phagolysosomal maturation. The aims in this proposal will help elucidate the mechanisms underlying the crosstalk between lysosomal maturation to inflammatory signaling and thus expand our knowledge of the mechanisms underlying gastrointestinal disorders such as IBD.